With the aggressive growth of cell phones and personal computers, the demand for lower cost, lighter weight and better efficiency battery chargers and small power standby supplies for personal computers is very high. Even though the linear power supply is low in cost, it becomes very difficult to compete with switching mode power supplies because of its heavy weight and low efficiency. The Flyback power converter is generally chosen among different switching mode topologies to meet this demand due to its simplicity and good efficiency. Over the years, various control ICs had been developed and used to build a Flyback power supply. FIG. 1 shows a typical prior art primary side controlled Flyback power converter. It consists of a transformer 201 which has three windings, primary switch 105, secondary-side rectifier 302, output capacitor 303 and a control IC 104. Resistor 101 and capacitor 102 provides the initial start-up energy for IC 104. Once the Flyback converter is stable, IC 104 will be powered by the auxiliary winding (with NΛ turns) of transformer 201 via rectifier 103. The output voltage is fed back to the primary side via the auxiliary winding, rectified and filtered by rectifier 107 and capacitor 110, and sensed by the voltage divider resistor 108 and 109. Resistor 106 senses the current flowing through the primary switch 105. IC 104 is a peak current mode PWM controller. Secondary resistor 301 represents the copper loss of transformer 201.
The circuit of FIG. 1 works well as long as the requirement of output voltage regulation is not stringent. Typically, a 10% load regulation with the loading from 10% to 100% of its rated maximum load can be met. However, the regulation becomes very poor when loading drops below 10% of its rated load. There are two factors causing the poor regulation: 1) the transformer copper loss varies with output current and input voltage; and 2) the auxiliary winding of transformer 201 contains an undesired resonant waveform when the Flyback converter operates in a discontinuous current mode (DCM). To achieve a tighter regulation requirement, others have used the prior art secondary side controlled Flyback converter shown in FIG. 2. This configuration generally meets a 5% load regulation over 0% to 100% of its rated load. In this circuit, the output voltage is sensed and an error signal is then fed back to the primary IC via the optical coupler. The main disadvantage of this circuit is higher cost. The additional components and a safety approved optical coupler add significant cost to the overall design. This additional material cost can be up to 10% more than the primary side converter shown in FIG. 1.
FIG. 2 shows a typical prior art secondary side controlled Flyback converter. In this circuit, the output voltage is sensed by the voltage divider resistor 305 and 307, and monitored by the secondary IC 306. The error signal is then fed back to the primary IC 104 via the optical coupler 202. The main disadvantage of this circuit is high cost. The IC 306 and the safety approved optical coupler 202 significantly increases the cost of type of converter. This cost increase can be as much as 10% of the overall material cost as compared to a primary side converter of FIG. 1.
In view of the foregoing, there is a need for a low-cost and effective control methodology that can regulate the output voltage of a Flyback converter from the primary side within good accuracy from 0% to 100% of its rated load.